Looking for advice on a SUB-4 hour Marathon 

Hi,
My name is Dan and I'm currently training for my second marathon in Philadelphia this November. I ran my first full this May in Pittsburgh with a time of 4 hours and 21 minutes. For my second marathon, I would really love to make a sub 4 hour marathon. I've been training according to the 18-week training plan posted on the Philadelphia Marathon website. However, I've learned today that my average pace starts to decline around 25km (15 miles-ish). I'm wondering if I should do more long runs or whether I should incorporate some speed workouts/other training - or perhaps both - more often.
I would love to take any suggestions on how to keep my pace steady for longer runs 👍🏻 Thank you for watching!